This invention relates to an implantable infusion pump. In particular, it relates to an infusion system utilizing a rechargeable, implantable constant pressure pump together with unique catheter arrangements to provide variable infusion rates.
Infusion pumps that are implanted in the body for delivery of an infusate, typically a medication or insulin to a selected site in the living body have reached the point of commercial and medical acceptance. Those devices generally fall into two categories. The first, are so-called constant flow devices which are used in a variety of medical applications, for example, to dispense chemotherapy drugs at a relatively constant flow rate. The technology represented by such constant flow devices is found in U.S. Pat. Nos. 3,731,681; 4,193,397; and 4,258,711. Those patents are representative of a number of other patented technology wherein an infusion apparatus relies on a liquid/vapor equilibrium to maintain constant pressure on the drug which flows through a capillary in order to maintain a constant flow rate. The device is implanted in the body to remain there for a prolonged period and is refilled with an infusate without the need for removal. Refilling is achieved by injecting additional infusate through a penetrable septum in the apparatus. The septum is located directly under the patient's skin and is sensed by tactile location. During the act of refilling of the apparatus with infusate, recharging of the apparatus' power supply also takes place since the liquid/vapor equilibrium is shifted by an increase in pressure in the infusate chamber.
A second class of devices are the so-called "programmable" devices which employ a valve or other device controlled by a programmer so that the dosage rate can be varied to suit the needs of a particular patient. An example is diabetes, where the quantity of medication, such as insulin, to be infused varies as a function of the requirements of the patient. Such fluctuations can occur on a daily basis or, randomly as a function of the ingestion of food. The patient receives a small continuous or basal dose to satisfy a particular steady state requirement as a function of the average amount of sugar in his blood following eating, when blood sugar levels rise dramatically, the infusion apparatus is programmed to dispense a larger (bolus) dose of insulin to offset the increased sugar level caused by the ingestion of a meal. The technology represented by such programmable devices is represented in U.S. Pat. Nos. 4,077,405; 4,443,218; 4,447,224; and 4,714,462.
While basic constant flow devices offer simplicity in operation and design, they do not allow for flexibility in dosage which is required by a number of treatment regimes. Conversely, while programmable pumps provide such flexibility, they suffer from difficulties in the areas of power consumption, overall life, and failsafe operation.
In order to address these two concerns, U.S. Pat. No. 4,496,343 relies on a constant pressure implantable infusion pump having attached to it an auxiliary chamber used for unrestricted bolus injections of medication and other fluids through the pump's infusion line, generally a catheter, to the delivery site. This auxiliary chamber or "bolus port" contains its own self-sealing septum which is accessed percutaneously via a hollow needle puncture. The auxiliary chamber is serially connected to the reservoir and, whether mounted internal or external to the pump, fluid flowing through the main reservoir passes through this auxiliary chamber prior to exiting through the delivery catheter.
Another alternative to programmable systems is found in U.S. Pat. No. 4,258,711. The system described therein employs two parallel reservoirs, each separately rechargeable and delivering via parallel flow paths a constant basal dosage or, a manual bolus. The bolus dose is held in an accumulator which is manually released by means of a button located externally which actuates an internally placed valve so that in addition to the constant flow basal dose, a "one-shot" bolus dose is delivered from the accumulator. While this system eliminates the complexities inherent in electronics which require an explant for changes of power source, nevertheless, the overall implantable system remains complex with bolus dose dependent on actuation of a manual valve coupled with multiple reservoirs. From the foregoing, it can be appreciated that the technology is advancing in an attempt to provide the necessary flexibility for varying drug delivery dosages without unnecessarily encumbering such systems with complex devices which increase the propensity for failure. Moreover, the utility of an auxiliary port has been demonstrated by commercial devices such as the Infusaid Model 400 to perform bolus injections, and to provide the ability to maintain catheter patency by periodic or high-pressure flushing. Additionally, such auxiliary ports provide additional flexibility with respect to the extraction of fluid samples such as blood from the delivery site. Such flexibility is inherent in a system which utilizes a single bolus port but cannot be accomplished in a system that employs valve technology such as the '711 patent.
One characteristic of utilizing a relatively unobstructed, serial catheter access device is that a bolus injection will force the infusate stored in the chamber through the catheter as the chamber volume is flushed. While such a characteristic is present in systems designed by the '343 patent, as noted, for certain drug therapy such as insulin, the therapy regime cannot tolerate a bolus drug injection per se and thus, a different flow configuration must be pursued which allows the use of an auxiliary port without the hazards of chamber flushing. Additionally, while the extraction of blood from the delivery site is desirable from a diagnostic standpoint, such may cause a build-up of blood products on the internal, relatively small, passages of the capillary tubing in '343. This in turn may lead to clotting and the blockage of flow. Obviously, alternative configurations are required.
Capillary tube restrictors which are used in such implantable devices have the advantage of providing a simple and low cost fluid restriction. Generally, large internal diameters (0.003-0.004 inches) are coupled with long lengths (sometimes in excess of thirty feet wrapped around the implantable device) to provide the necessary restrictions with reduced risk of contamination due to plugging and a large degree of freedom insofar as trimming to a desired flow rate. The overall length and the internal diameter are chosen to provide low shear rate, laminar flow in order to reduce the stress on a specific drug solution which may be sensitive to shear. However, such long lengths of capillary tubing increase the space requirement and overall weight of the system. In the context of an implantable device, these two properties are significant drawbacks in that they limit areas where implantation may occur and provide a degree of user discomfort. Moreover, the actual measuring, calibration and trimming of long tube lengths is labor intensive and requires a considerable amount of time.
U.S. Pat. No. 3,951,147 relating to an implantable infusate device describes a technique for replacing the long length of catheter tubing by employing a flow controller utilizing a large bore diameter tubing having placed therein a resistance wire. By properly sizing the diameter and length of the resistance wire, flow control can be maintained. The '147 patent therefore proceeds by reducing the overall length of the capillary tubing through the use of a shorter tubing having placed therein a predetermined length of wire to achieve the necessary flow characteristics of the overall device. However, no provision is made for extending the resistance wire external to the capillary for adjustment during manufacture or adjustment during use (i.e. in-vivo).